The balance sensing functionality of the brain is developed based on neural signals from the vestibular structures of the inner ear, one on each lateral side of the body. As shown in FIG. 1, each inner ear vestibular labyrinth 100 has five sensing organs: the ampullae 108 of the three semi-circular canals—the horizontal (lateral) canal 103, the posterior canal 104, and the superior canal 105—which sense rotational movement, and the utricle 106 and the saccule 107 in the vestibule 102, which sense linear movement.
FIG. 2 shows anatomical detail within a vestibular canal ampulla 108 which is connected at one end to the canal 206 and at the other end to the vestibule 205, and which contains endolymph fluid. The vestibular nerve endings 204 connect to the crista hair cells 203, the cilia ends 202 of which are embedded in the gelatinous cupula 201. When the head moves, the endolymph fluid within the ampulla 108 defects the cupula 201, generating a sensing signal in the vestibular nerve endings 204 that is interpreted by the brain as the sense of balance.
Unfortunately some people suffer from damaged or impaired vestibular systems. Such vestibular dysfunction can cause balance problems such as unsteadiness, vertigo and unsteady vision. Delivery of electrical stimulation to the vestibular system is currently under research to treat patients suffering from vestibular related pathologies. Experimental results indicate that electrical stimulation of the vestibular system has the potential to restore vestibular function, at least partially. See, e.g., Rubinstein J T et al., Implantation of the Semicircular Canals With Preservation of Hearing and Rotational Sensitivity: A Vestibular Neurostimulator Suitable for Clinical Research, Otology & Neurology 2012; 33:789-796 (hereinafter “Rubinstein”); Chiang B et al., Design and Performance of a Multichannel Vestibular Prosthesis That Restores Semicircular Canal Sensation in Rhesus Monkey; IEEE Trans. Neural Systems and Rehab Engineering 2011; 19(5):588-98 (hereinafter “Merfeld”); and Gong W at al., Vestibulo-Ocular Responses Evoked Via Bilateral Electrical Stimulation of the Lateral Semicircular Canals, IEEE Transactions On Biomedical Engineering, Vol. 55, No. 11, November 2008 (hereinafter “Della Santina”); all incorporated herein by reference.
One challenge in developing a vestibular implant is the design of a device-to-body interface, the stimulation electrode. Such a vestibular stimulation electrode needs to selectively stimulate at least one of the vestibular nerve branches for the vestibular canal ampullae. Typically insertion of the stimulation electrode is though the semicircular canal. The stimulation electrode should be located as close as possible to the nerve fibers of the hair cells in the ampulla crista without damaging them.
Currently, different research groups are working on the development of different vestibular implants, with intra-labyrinthine stimulation approaches being of interest for the present purposes. The Merfeld group has described different electrode types using simple wires as stimulation electrodes. This research group has also described the development of polyimide thin film electrodes for a vestibular prosthesis, though no published data with results is available of this kind of electrode. See Hoffmann K P et al., Design of Microelectrodes for a Vestibular Prosthesis, BMT 2011 Rostock, Germany (incorporated herein by reference).
The Rubinstein research group published details of a vestibular stimulation electrode in the previously cited Rubinstein article, as well as in U.S. Patent Publication 2012130465 and PCT Patent Publication WO 2010138915 (incorporated herein by reference). Their stimulation electrode, as shown in FIG. 3, has a relatively small diameter to prevent compression of the membranous canals using “soft surgery” techniques. They claim to have developed a vestibular stimulation electrode that allows post-surgical preservation of the natural function of the vestibular system.
The Della Santina research group published details of their stimulation electrode in the previously cited Chiang reference, as well as in U.S. Pat. No. 7,647,120 and PCT Patent Publication WO 2011088130 (FIG. 4) (all incorporated herein by reference). Their prosthesis is being developed for treatment of bilateral vestibular hypofunction (BVH) for which there is no absolute need to preserve natural vestibular function. The research and development strategy here accepts compression or other trauma to the membranous labyrinth in order to get the stimulation electrodes closer to the respective nerve branches. Since the membranous duct fills out almost the entire ampulla, it is virtually impossible to reach the crista without compressing or otherwise traumatizing the membranous canals. FIG. 4 shows this stimulation electrode which uses three active branches each with three electrode contacts. Two of the electrode branches are combined together to form a double forked structure. Two reference electrodes also are used: a “distant reference” for placement on the skull under the temporalis muscle, and an “intralabyrithine reference” for placement inside the common crus (the common part of the superior and posterior canals).